During a telephone call, speech from a remote user may be provided to a local user via a loudspeaker, and speech from the local user may be picked up by a microphone and transmitted to the remote user. In some cases, such as in teleconferencing applications, the loudspeaker may be located relatively close to the microphone and the audio signal picked up by the microphone may include audio content originating from the remote user. Such audio content may be perceived by the remote user as echo, and is preferably suppressed or cancelled from the audio signal sent to the remote user.
FIG. 1 shows a setup for reducing this echo via so-called acoustic echo cancellation. Speech from the remote user is played back by a loudspeaker 101 and speech from a local user 102 is picked up by a microphone 103 together with the speech from the remote user played back by the loudspeaker 101. A filter 104 is provided for modeling or approximating the system formed by the loudspeaker 101, the microphone 103 and the acoustic environment in which the loudspeaker 101 and the microphone 103 are arranged (e.g. acoustic properties of a room in which the loudspeaker 101 and the microphone 103 are arranged, including the distance between the loudspeaker 101 and the microphone 103 and distances to walls which may reflect sound). The filter 104 is applied to a reference signal 105 including the speech from the remote user. A residual signal 106 is formed by subtracting the filtered reference signal 107 from a response signal 108 provided by the microphone 103 in response to playback by the loudspeaker 101 of the reference signal 105. The subtraction serves to cancel audio content originating from the remote user in the response signal 108 provided by the microphone 103. The residual signal 106 is transmitted, e.g. after additional processing, to the remote user. When the local user 102 is silent, the residual signal 106 is indicative of the modeling error of the filter 104. Hence, when the local user 102 is silent, the filter 104 is adjusted based on the residual signal 106 so as minimize energy or power of the residual signal 106, and thereby to improve the acoustic echo cancellation.
The adaptive filtering provided by the filter 104 may for example be performed by filters in respective frequency subbands. By employing separate filters in the respective subbands, different update step sizes for the filters may for example be employed in the different subbands, depending on the energy of the reference signal 105 in the respective subbands. This may improve the adaption rate of the filter 104.
When employing subband filtering to perform acoustic echo cancellation, downsampling may be employed to reduce computational complexity. This may cause aliasing of audio content from one frequency subband into neighboring subbands. Such aliasing may cause audible echo to persist in the residual signal 106 even if acoustic echo cancellation is performed in each subband. An approach to handle such aliasing is described in the paper “Adaptive Filtering in Subbands with Critical Sampling: Analysis, Experiments, and Application to Acoustic Echo Cancellation” by A. Gilloire and M. Vetterli in IEEE Transactions on Signal Processing, vol. 40, no. 8, August 1992. This approach is illustrated in FIG. 2. For a given subband k, a first filter 201 is applied to the reference signal in that subband k, and second 202 and third 203 filters are applied to the respective neighboring subbands k−1 and k+1 of the reference signal. A residual signal 204 is computed by summing the three filtered subbands signals 205, 206 and 207 and subtracting the sum 208 from a response signal 209 for that frequency subband received from a microphone. The filters 201, 202 and 203 may be adapted to provide acoustic echo cancellation by minimizing the energy or power of the residual signal 204. The second filter 202 and the third filter 203 model aliasing between the given subband k and the neighboring subbands k−1 and k+1 and may improve echo cancellation in at least some applications. Analogous computations may be performed for the other frequency subbands.
Adaptive audio filters similar to those applied for acoustic echo cancellation may be employed also for other purposes. A system providing an output audio signal in response to a reference audio signal may for example be modeled or approximated via such adaptive filters. In other words, filters approximating the impulse response (or transfer function or frequency response) provided by such a system may be provided via such adaptive filtering schemes. For example, adaptive filters may be employed for modeling or approximating the response of a loudspeaker to a speaker feed.
Design of adaptive filtering schemes may for example include considerations relating to the accuracy of the approximation provided by the adaptive filters, the adaption rate of the adaptive filters (i.e. the ability to track changes in the system which is to be modeled or approximated) and/or the computational complexity of the adaptive filtering scheme.
All the figures are schematic and generally only show parts which are necessary in order to elucidate the invention, whereas other parts may be omitted or merely suggested.